This invention relates generally to printing and die cutting apparatus and more specifically to rotary printing and soft anvil die cutting apparatus for the corrugated paperboard industry that uses flexible printing dies on a rotating cylinder to transfer ink to the surface to be printed and serrated edge cutting rules on a rotating cylinder acting against a soft anvil roll to die cut the printed sheets. Such apparatus is generally called flexographic printer die cutters which apparatus, among other things, feeds individual sheets of corrugated paperboard past a rotating printing die to print indicia on the sheets and past a rotating die cylinder to die cut the sheets.
One problem associated with such machines has been the mounting of the printing die to the print cylinder, particularly, quick mounting of the printing die so as not to delay set-up of the machine during a change from one sheet size to another and/or a change in the indicia to be printed on the sheet. The printing dies themselves are usually made of a type of rubber or plastic mounted on a backing sheet of heavy paper, rubber, or plastic material. The backing sheet is flexible so that it can be wrapped around the print cylinder.
An early means of mounting the printing die to the cylinder was merely to staple the backing sheet to a wooden covering on the cylinder. This left much to be desired since the staples eventually ruined the wooden covering and the backing sheet making it necessary to recover the cylinder and, more often, to replace the backing sheet or the complete die. In addition, if the printing die was not placed properly on the cylinder, it had to be removed and repositioned thus resulting in additional set-up time.
Since then, various means have been employed to mount the printing die to the print cylinder. One such means required a rigid strip on each end of the die backing of which one was captured in an immovable slot in the print cylinder and the other captured in a movable slot. The movable slot included a hinged portion movable in a direction away from the immovable slot so as to tension the backing sheet. The hinge portion was moved by a pneumatically expandable tube. The die backing had to be made quite precisely and tube failures were not uncommon.
Another means uses rigid U-shaped hook strip on one end of the backing sheet that hooks in a mating U-shaped slot in the print cylinder, the so-called "Dorr" system. The other end of the sheet is made similar to a roll-up window blind with the roller placed in a slot in the print cylinder. A special tool is used to wind up the roller to tension the backing sheet to hold it tightly against the cylinder. Making the backing sheet is quite complex and expensive. In addition, relatively considerable time is required to roll up the backing sheet in the cylinder.
Probably the most popular means in current use is the so-called "Matthews" system which includes a U-shaped hook strip on one end of the backing sheet that hooks in a mating U-shaped slot in the print cylinder. A number of elastic straps are fastened to the other end of the backing sheet. Each strap has a U-shaped hook that hooks into a mating U-shaped slot in the cylinder. The elastic straps tension the backing sheet tightly against the cylinder. Making the backing sheet with straps is fairly complex and expensive and some time is required to fasten each individual strap in the slot in the cylinder.
All of the above means rely on tension to hold the backing sheet tightly against the printing cylinder. However, tension is not always necessary as evidenced by the so-called "Magna Graphics" system which uses a backing sheet of ferrous material held to the cylinder by permanent magnets embedded in the cylinder's surface. Thus, both the backing sheets and the cylinder are expensive to make. In addition, any loose ferrous materials around the press are attracted to the cylinder which can damage the ink transfer roll that transfers ink to the printing die.
One problem associated with rotary die cutters is that considerable time is required to mount the cutting die on the die cylinder. It should be recognized that a different die is needed for each order of sheets to be die cut because the sheet size and configuration varies from order to order. Sometimes several orders are run in the course of single shift thus requiring a die change prior to each order. When several minutes are required to change each die, this amounts to many minutes, and sometimes hours, of downtime each day.
Rotary die cutting is often done at the same time that the sheets are printed on the adjacent printing apparatus so that the sheets need be fed but once into the printing and die cutting apparatus. Thus, if steps are taken to reduce the amount of set-up time needed to change the printing dies, then a means of rapidly mounting die cutting dies is especially needed to reduce the total set-up time.
Die cutting dies include steel rule having serrated cutting edges permanently mounted in a curved plywood blanket that is mounted on the die cylinder. An example of such die rules and blanket is shown in U.S. Pat. No. 26,192 which shows a small hand-hole die mounted on a small plywood blanket. However, for larger die cuts to, for example, cut out a complete sheet which is more common, the die may completely cover the die cylinder from end to end and its entire circumference. In this event, the plywood blankets are made the length of the die cylinder and one-half its circumference to permit two halves to be mounted on the cylinder to form a substantially completely annular die.
The plywood blanket is substantially rigid but may warp slightly due to inherent stresses in the wood. Thus, the conventional means of mounting them is to bolt each half onto the die cylinder. The cylinder is provided with pre-tapped bolt holes and the blanket is made with bolt clearance holes in alignment with the tapped holes. Drawing down the bolts outward from the center of the blanket overcomes any warp in the die blanket so that it conforms to the curvature of the die cylinder. A great many bolts are required to hold the blanket in place which accounts for the extraordinary amount of time required to mount them.
It will be well understood by those skilled in the art that the length of the die blankets will frequently be less than the entire length of the die cylinder and often only a single blanket of one-half or less of the circumference is required to die cut a particular blank.
Accordingly, an object of the present invention generally is to provide a rapid mounting means for printing and die cutting dies.
Another object of the present invention is to provide a die mounting apparatus that is simple and inexpensively constructed that will hold printing and die cutting dies tightly against the cylinders upon which they are mounted.
A more specific object is to provide a die mounting apparatus utilizing subatmospheric pressure to hold the die against the cylinder and to do so in a manner whereby subatmospheric pressure in the cylinder is applied only to those areas beneath the die.